Download Reactivity Series and Corrosion

Reactivity Series and Corrosion
Introduction – Teacher notes (Strong GCSE or A-Level)
For this piece of work, it is assumed that a basic introduction to corrosion has been
given and that pupils are aware of the following:
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Rusting applies to iron only.
Corrosion is an (electro)chemical process.
Some metals are more reactive than others – reactivity series / displacement
reactions.
If two dissimilar metals are joined in a corrosive environment, the most reactive
metal corrodes; the least reactive metal does not.
Structural metals can be protected by coating them / connecting them with a
more reactive metal – sacrificial protection.
Differences in reactivity between two dissimilar metals can also be used in an
electrochemical cell as a portable source of electricity.
The driving force for the electrochemical reaction can be measured in terms of
the emf of a cell using a voltmeter when no current flows between the
electrodes.
The following work can be set as a class practical exercise or as an investigation. There
are many difficulties with measuring the emf of the cells which must be tackled by the
pupils allowing for detailed discussion of accuracy, reproducibility and experimental
error. However, if this can be done then there is the possibility of good marks.
The major problem is in obtaining reproducible results; since the measured voltage will
not be a true emf (some current will flow). There is a major problem with fluctuating
results due to many causes (changes on the electrode surface, diffusion, convection and
polarisation). On top of this there is the problem of the measured potential decaying
along an exponential curve.
Pupils must realise that this is part of the experimental problem which they have to
overcome and is not because apparatus does not work or they have “done something
wrong”.
A technique to obtain acceptable results would include…
 Always start with a fresh electrode, preferably abraded just before use (fresh
abrasive paper to avoid particles of another metal being scratched into the
surface.
 Connect electrodes before inserting into the solution.
 Leave electrodes for a predetermined time before taking a reading, e.g. 10 or 15
sec.
 Always record which metal is connected to negative terminal of the voltmeter.
This exercise takes the form of a scientific investigation which assesses the ability to
measure data with an awareness of experimental error and to evaluate those results.
There is scope for more than one investigation along these lines.